1. Field of the Invention
The present invention relates to a process of blowing hollow containers such as bottles or the like from closed-end parisons and particularly to an injection/stretch/blow molding process for forming a predetermined concavo-convex pattern in the bottom of a hollow container.
2. Description of the Related Art
There are well-known injection/stretch/blow molding techniques which are utilized to blow hollow containers such as bottles from synthetic resin materials such as polyethylene terephthalate (PET). The hollow containers thus blown are transparent with surface gloss as in glass bottles and superior in various properties such as resistance to shock, air blocking, handling for design, mass-productivity and so on. For such reasons, the hollow blown containers are currently used for various fluid materials such as cold drinks, powdered detergents and the like.
However, such hollow blown containers are easily deformable since the wall thickness thereof is small. For example, if they are used as pressurized liquid containers for carbonated beverages and beers, the walls and particularly the base portions in the containers are easily deformable due to the internal pressure from the contents thereof. The base portions of the hollow blown containers are required to be strengthened sufficiently to resist the internal pressure described as well as drop test.
In order to overcome such problems, it has been proposed that the bottoms of the hollow blown containers were formed with concavo-convex patterns and particularly strengthening ribs, as in Japanese Laid-Open Patent Application 79238/1980.
This proposal uses a core utilized to injection mold a parison and having a concavo-convex pattern formed therein. The core is used so as to provide a closed-end parison having its bottom in which strengthening ribs are formed by the concavo-convex pattern during molding.
However, this proposal can be applied only to the so-called cold parison technique, but not to the hot parison technique in which immediately after a parison has been injection molded, it is blown utilizing its potential heat.
In other words, the injection/stretch/blow molding technique for blow molding hollow containers from closed-end parisons frequently utilizes the hot parison technique for blow molding hollow containers by the use of potential head in parisons immediately after they have been injection molded.
In the hot parison technique, the distribution of temperature in the hot closed-end parison highly influences the wall thickness of a hollow container to be blown.
It is thus required to injection mold a hot closed-end parison without creation of irregularity in temperature.
The injection molding of the closed-end parison is accomplished by injecting a synthetic resin material such as PET into a small mold cavity which is surrounded by a core and mold halves. If the core so used is formed, at its tip, with a rib-shaped concavo-convex pattern in the prior art, the flow of injected material will be disturbed to provide uneven wall thickness in the resulting parison, leading to an irregularity in temperature. Accordingly, such a technique could not be applied to the hot parison technique.
When it is particularly desired to produce a PET bottle having its very complicated configuration, a closed-end parison itself used therefor frequently has a complicated configuration. If such a closed-end parison is to be injection molded, it is required to control the injection molding procedure such that the flow of PET in the mold cavity will not be disturbed as much as possible, without creation of irregularity in temperature. It is not absolutely acceptable to provide a rib-shaped concavo-convex pattern on the tip of the core used to form the mold cavity.